This patent specification relates to optical amplifiers. More specifically, it relates to a semiconductor optical amplifier that amplifies an optical signal using energy from one or more nearby and/or intersecting laser cavities.
Efforts continue toward improving the performance of semiconductor optical amplifiers (SOAs) to levels that would allow their increased use in optical communications systems, such as in replacing bulkier and more expensive erbium doped fiber amplifiers (EDFAs). This includes making SOAs with higher power and lower noise characteristics, as well as SOAs having reduced pattern dependent amplitude fluctuations that can cause crosstalk among different channels in a wavelength division multiplexed (WDM) optical communications system. An SOA generally resembles a semiconductor laser structure, except that the end mirrors are replaced by antireflection coatings. In such devices the product of the gain and the reflectivity is less than one so that the device does not oscillate. Rather, the device is used to amplify an incoming optical signal as it passes through the device. Such devices are often called traveling wave amplifiers, which highlights the fact that the optical signal does not pass back and forth within the device, but merely passes through it essentially only once.
One particularly appealing strategy for reducing pattern dependent amplitude fluctuations in an SOA is to place a transverse laser across the SOA such that the laser""s gain medium and the SOA""s signal gain medium share an overlapping region. The lasing cavity is operated above threshold and the gain of the laser is clamped to overcome the losses of the cavity. As used herein, a laser cavity is gain-clamped and lasing when it is excited by a bias current greater than a threshold current. When the transverse laser is gain-clamped, gain along the SOA signal path is stabilized. The transverse lasing enhances the establishment and maintenance of a population inversion in the overlapping region, resulting in both increased saturation power and a decreased gain recovery time due to increased photon density in the laser cavity. Advantageously, independent lasing only builds up in the transverse direction and does not corrupt the quality of the amplified signal. See generally U.S. Pat. No. 5,436,759; Francis, et. al., xe2x80x9cA Single Chip Linear Optical Amplifier,xe2x80x9d IEEE Optical Fiber Communication Conference, Anaheim, Calif. (2001); Ser. Nos. 09/972,146 and 10/006,435, supra.
In U.S. Ser. No. 09/972,146, supra, a ballast-powered SOA is described comprising a signal waveguide that guides an optical signal along a signal path that intersects with one or more transverse laser cavities, the gain medium of the signal waveguide being integral and coextensive with a gain medium of the transverse laser cavities at regions of intersection therebetween. Gain-stabilized operation is achieved when the transverse laser cavities are biased above threshold. Because it is the lasing fields of the transverse laser cavities that supply the energy for amplifying the optical signal, the transverse laser cavities may be termed ballast lasers, and the SOA may be termed ballast-powered. Successive ballast lasers are separated along the signal path by connecting zones having a higher electrical resistivity than the ballast lasers, providing a measure of electrical isolation therebetween and reducing parasitic lasing modes among them. The ballast lasers are preferably provided with separate bias currents for precise control of gain along the signal path. Additionally, one or more of the ballast lasers may be segmented in the direction of the lasing field into multiple segments with separate electrical contacts. Through careful selection of the bias currents applied to the different segments of successive ballast lasers, the SOA device can be adapted for use in several different applications and/or can achieve improved operating characteristics.
In U.S. Ser. No. 10/006,435, supra, a ballast-powered SOA is described comprising a signal waveguide and one or more transverse ballast lasers, each ballast laser having a gain medium that lies outside the signal waveguide rather than being coextensive with the signal waveguide. The gain medium of the ballast lasers is sufficiently close to the signal waveguide such that, when the gain medium is pumped with an excitation current, the optical signal traveling down the signal waveguide is amplified by an evanescent coupling effect with the ballast lasers. When the gain medium is sufficiently pumped to cause lasing action in the ballast lasers, gain-clamped amplification of the optical signal is achieved. Additional features relating to segmented laser cavities, separate pumping of laser cavity segments, DFB/DBR gratings, current profiling to improve ASE noise performance, coupled-cavity lasers, avoidance of injection locking effects, manipulation of gain curve peaks, integration with a tunable vertical cavity coupler, integration with a photodetector, and integration with an RZ signal modulator are also described. These additional features are applicable to both (i) the SOA with coextensive ballast laser coupling as introduced in Ser. No. 09/972,146, and (ii) the SOA with evanescent ballast laser coupling as introduced in Ser. No. 10/006,435, supra.
Amplified spontaneous emission (ASE) is a troublesome noise source in conventional SOAs. ASE arises from random, spontaneous energy state drops in a small fraction of the excited carriers of the gain medium. Light emitted as a result of these energy state drops is generally random in direction and wavelength. Some of this light will be emitted in the direction of signal propagation and will therefore be amplified as it propagates, resulting in output background noise signal similar to white noise. For good performance, it is generally desirable to keep the amplified signal levels at least 10 dB higher than ASE levels.
Relative intensity noise (RIN) is a troublesome noise source in conventional semiconductor lasers. RIN refers to a random intensity fluctuation in the output of the laser. As the laser operates, new spontaneous emissions occur and some of them can resonate within the cavity and be amplified. This causes some fluctuation in the output power. Of course, RIN noise is not an issue in conventional SOAs because they are not lasers and have no lasing action. However, in ballast-powered SOAs the RIN noise experienced in the transverse ballast lasers can couple into the SOA output signal by virtue of its relationship to the photon density in the laser cavities.
From a signal processing perspective, the effects of RIN noise are fundamentally different from the effects of ASE noise. In particular, whereas ASE noise simply adds to the optical signal as it passes through the SOA, RIN noise actually modulates the optical signal by virtue of the gain fluctuations it imposes. The frequency spectrum of the relevant gain fluctuations generally extends from very low frequencies up to about 16 GHz or more, as showed in the sample RIN spectrum 10 of FIG. 1. The RIN spectrum 10 experiences very low levels (a xe2x80x9cnoise floorxe2x80x9d) in the 0.01-0.1 GHz range, but begins to rise more quickly in the low-GHz range, and then reaches a RIN peak 12 at a relaxation oscillation frequency (ROF) 14 of the laser, a consequence of the laser""s intrinsic resonance. The RIN spectrum 10 then falls off again as the frequency increases further beyond the relaxation oscillation frequency (ROF) 14. The relaxation oscillation frequency (ROF) 14 is usually somewhere between 3 and 16 GHz or more depending on the laser design and the photon density.
The relaxation oscillation frequency (ROF) 14, the shape of the RIN peak 12, and the overall location and vertical position of the RIN spectrum 10 depend on the particular characteristics of the laser such as facet reflectivity, material characteristics, etc. Importantly, however, changes in the relaxation oscillation frequency (ROF) 14, the RIN peak 12, and the overall RIN spectrum 10 occur as the specific operating point of the laser changes. Generally speaking, the relaxation oscillation frequency (ROF) 14 shifts to the right as the current density through the laser""s gain medium is increased, while the height of the overall RIN spectrum 10 and the severity of the RIN peak 12 decrease with increased laser output power. See generally Agrawal, Semiconductor Lasers, 2nd ed., Van Nostrand Reinhold Publishers (1993) at pp. 258-297.
It would be desirable to provide a ballast-powered semiconductor optical amplifier (SOA) for use in an optical communications system, wherein bit errors caused by relative intensity noise (RIN) in the ballast-powered SOA are maintained at acceptably low levels.
It would be further desirable to provide such a ballast-powered semiconductor optical amplifier (SOA) in which amplified spontaneous emission (ASE) is also maintained at acceptably low levels.
According to a preferred embodiment, a ballast-powered semiconductor optical amplifier (SOA) apparatus and related methods are provided for amplifying an optical signal having a first modulation rate, the SOA comprising a signal waveguide that guides the optical signal along an optical signal path, the SOA further comprising a ballast laser positioned with respect to the optical signal path such that the optical signal is amplified using energy from the lasing field of the ballast laser, wherein the ballast laser is biased by an excitation current sufficient to cause relative intensity noise (RIN) in an SOA output to be at acceptably low levels. More particularly, the excitation current is maintained at a level greater than a predetermined RIN threshold current, the RIN threshold current corresponding to an excitation current that yields a relaxation oscillation frequency (ROF) sufficiently greater than the first modulation rate such that a RIN spectrum value at the first modulation rate is equal to a predetermined tolerance amount above a low-frequency RIN noise floor. In one preferred embodiment this predetermined tolerance amount is at least 6 dB/Hz. The excitation current of the ballast laser must also be greater than a lasing threshold sufficient to cause lasing in the ballast laser.
In another preferred embodiment, the ballast laser of the SOA is biased by an excitation current sufficient to result in an SOA output saturation power greater than a nominal output saturation power of the SOA. In particular, the excitation current is maintained at a level greater than a predetermined saturation threshold current, the saturation threshold current corresponding to an excitation current that yields an output saturation power equal to the nominal SOA saturation output power. In another preferred embodiment, the excitation current of the ballast laser is maintained at a level not less than the greatest of (i) the lasing threshold current, (ii) the RIN threshold current, and (iii) the saturation threshold current.
According to another preferred embodiment, the ballast laser is segmented into multiple segments including a main segment whose lasing field energy couples with the optical signal and an auxiliary segment, the main segment and auxiliary segment being optically contiguous but electrically isolated and provided with separate excitation currents. The auxiliary segment excitation current is set at a level corresponding to a desired gain of the SOA, while the main segment excitation current is maintained at a level not less than the greatest of (i) a predetermined lasing threshold current sufficient to cause lasing in the ballast laser for that gain level, (ii) a predetermined RIN threshold current that yields an ROF sufficiently greater than the first modulation rate such that a RIN spectrum value at the first modulation rate is equal to a predetermined tolerance amount above a low-frequency RIN noise floor for that gain level, and (iii) a predetermined saturation threshold current corresponding to an excitation current that yields an output saturation power equal to the nominal SOA saturation output power.
According to another preferred embodiment, the SOA comprises multiple ballast lasers positioned along the optical signal path, each ballast laser being associated with a corresponding amplifier stage along the signal path. The multiple ballast lasers include a first ballast laser associated with a first amplifier stage having a first signal gain per unit distance therethrough, and further include a second ballast associated with a second amplifier stage having a second signal gain per unit distance therethrough, the first amplifier stage being located nearer to an SOA signal input than the second amplifier stage, wherein the first signal gain per unit distance is greater than the second signal gain per unit distance. For a fixed amount of signal amplification collectively yielded by said first and second amplifier stages, amplified spontaneous emission (ASE) noise in the SOA output is reduced as compared to a configuration in which said first and second signal gains per unit distance are the same.
According to another preferred embodiment, each amplifier stage positioned nearer the SOA signal input yields a signal gain per unit distance greater than that of each amplifier stage positioned farther from the SOA signal input. In another preferred embodiment, each of the multiple ballast lasers comprises multiple segments including a main segment and an auxiliary segment, the auxiliary segment excitation current being set to a level corresponding to a desired gain for that amplifier stage, the main segment excitation current being maintained at a level not less than the greatest of (i) a predetermined lasing threshold current sufficient to cause lasing in that ballast laser for that amplifier stage gain level, (ii) a predetermined RIN threshold current that yields an ROF sufficiently greater than the first modulation rate such that a RIN spectrum value at the first modulation rate is equal to a predetermined tolerance amount above a low-frequency RIN noise floor for that amplifier stage gain level, and (iii) a predetermined saturation threshold current corresponding to a main segment excitation current that yields an output saturation power for that amplifier stage equal to the nominal saturation output power for that amplifier stage for that amplifier stage gain level.